As described in “An Anatomic Study of the Lumbar Plexus with Respect to Retroperitoneal Endoscopic Surgery”, SPINE Vol. 28, Number 5, pp. 423-428, 2003, by Takatomo Moro et al., nerve mapping can be critical during surgery, such as endoscopic surgeries, to treat various lumbar spine diseases. For example, retroperitoneal endoscopic surgery has been applied to anterior interbody fusion for disc herniation, anterior decompression and interbody fusion for burst fracture and discectomy for extreme lateral disc herniation. However, to perform such a surgery the psoas major muscle may be separated. Doing so generates a risk of injury to the lumbar plexus or nerve roots. Accurate nerve mapping is thus critical to avoid trauma to nerves, such as those exposed during dissection of the psoas muscle.
Various methods exist for monitoring nerves (i.e., nerve mapping) during surgical procedures. Such methods may determine when an electrified instrument is approaching a nerve. Specifically, a known current is communicated to the instrument. As the instrument is manipulated within or on the patient, the current may evoke a muscular response. The muscular response is recorded and an auditory and/or visual signal is produced which alerts the user, such as a physician, that the instrument is considered to be near the nerve coupled to the responsive muscle.
However, such methods do not provide the user with the location of the nerve evoking the muscular response. Instead, they only suggest a nerve is in the vicinity of the instrument. Also, to record the muscular response the responsive muscle must not be numbed or temporarily paralyzed with, for example, neuromuscular blockade agents. As a result, the user may have difficulty advancing the instrument through a muscle that is not only active (i.e., not paralyzed), but which may even be contracting to resist or fight the user's efforts.